Structure-property relationships in segmented copolymers

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Virginia Polytechnic Institute and State University


The structure-property relationships for various segmented copolymers were investigated. A number of mechanical and thermal characteristics were determined. The morphology was characterized by SAXS with respect to size and dispersion of domains; degree of phase separation and the domain interfacial thickness.

a. Novel segmented copolymers were synthesized using amino terminated polydimethylsiloxane oligomers as the soft component and various hard component. lt was found that the performance of these copolymers was affected by the varying the hard segment linkages, hard segment content, soft segment MW, stiffness of the siloxane backbone, and amount of chain extender. Two-phase nature of these copolymers was verified by dynamic mechanical, thermal, and SAXS studies. The phase separation was found to occur in these copolymers with as little as 6% HS. These materials displayed behavior similar to the segmented polyurethanes and were found to be superior to the unfilled silicone elastomers. The final materials were used as reasonable models for investigating various methods for determining the interfacial layer thickness between the hard and soft phase. Specifically, due to the fact that there is no hard segment length distribution as is the usual case for segmented urethanes, these materials have some degree of model characteristics. Utilizing Porod's law, and appropriate analysis, both positive and negative deviations were found in the systematic series of copolymers with the degree of positive and negative character dependent upon composition. Negative deviations were accounted for in terms of a finite interfacial thickness which turned out to be relatively small as anticipated, while the positive deviations arose due to isolated hard segments that reside within the soft segment matrix, concentration fluctuations. ln calculating the interfacial thickness, several methods were applied and in general, close agreement was obtained. Finally, correlation function analysis in conjunction with determination of the coherent Porod lengths, etc. were determined and discussed accordingly.

b. The structure-property behavior of novel 'water extended' segmented polyurethane-urea copolymers was also investigated. These copolymers were synthesized by utilizing the dehydration characteristics of tertiary alcohols at sufficiently high temperature in weak acidic medium. Mechanical, thermal, dynamic mechanical, and x-ray experiments were carried out to characterize the morphology and properties of these segmented copolymers of systematically varying hard segment content, soft segment MW, block length, and hard segment type. lt was observed that these properties depended primarily on the degree of order in the hard domains and the order could be improved by increasing either the HS content at constant soft segment MW or soft segment molecular weight at the same HS content. The results obtained for these materials were compared with those from conventional polyurethanes to investigate the effect of intermolecular hydrogen bonding on molecular arrangement.

c. The final series of segmented copolymers studied were based on polysulfone and polydimethylsiloxane, synthesized by solution polymerization. lt is shown that by varying the length of the segments for each phase and their relative content, it is possible to alter the mechanical and thermal characteristics. The mechanical response was also influenced according to which phase is predominantly continuous. In addition to dynamic mechanical and SAXS measurements, the evidence of the two-phase structure was obtained by TEM.